MAN AND THE VERTEBRATE SERIES. 785 



water which surrounds their food must blunt the senses of taste and 

 smell, and there is little besides water contact to develop touch. 



Also the conditions surrounding attack and escape are here 

 greatly simplified. The water presents no lurking-places, no am- 

 bush, except to the inactive dwellers upon the sea-bottom. And, by 

 relieving its inhabitants from the effects of gravitation, it renders 

 rapid motion easy, with slight muscular exertion. Thus the easiest, 

 most natural, and most effective means of assault and defense is by 

 swift powers of swimming. 



The same principle holds good in the case of the dwellers of the 

 air. There is here no lying in wait for prey nor hiding from assault. 

 Flight is the most effective and most ready means of attack and es- 

 cape, and the only one available in the fields of open air. Thus purely 

 water and purely air animals fail to develop a variety of resources in 

 this particular. 



Moreover, as fish are at a disadvantage from their imperfect oxida- 

 tion, so are birds at a disadvantage from the effects of gravitation. 

 The weight of fishes is almost or entirely supported ; that of land- 

 animals partly supported ; that of birds almost unsupported. Gravita- 

 tion in them, then, must be mainly overcome by muscular exertion. 

 Thus a large proportion of their life-force is exhausted by the effort 

 to sustain themselves against the constant downward pull of gravity. 

 This places them at a disadvantage with animals capable of using their 

 forces for more varied pui'poses. 



Indeed, we find, both in fishes and in birds, a tendency to avail 

 themselves to some extent of the advantages which the land-surface 

 gives. And those that most display this tendency comprise the spe- 

 cies of most varied resources, and with the greatest degree of func- 

 tional adaptation. Many species of birds, in fact, have found safety 

 so much more assured on the land, that they have lost, first their in- 

 stinct, and then, in some cases, their power of flight, and have become, 

 virtually, sm-face-animals. 



We may descend to the invertebrate world for one marked in- 

 stance of this. The ants are acknowledged to be the highest of all 

 insects in functional development. Their eyes, for instance, are more 

 simplified than those of other insects ; and, if we class mental attri- 

 butes as nerve-functions, their claim is indisputable. Yet they have 

 forsaken the air and taken to the earth in preference, the females cast- 

 ing off their wings as if in scorn, after a temporary use of them. 



Of all the fields of life for the varied display and functional sepa- 

 ration of the animal forces, we are thus brought to the land-surface as 

 obviously the best. Here an abundant supply of oxygen assures vital 

 activity ; the firm ground largely supports the weight, and releases 

 the muscular powers for employment in other directions ; the abun- 

 dance and variety of vegetable food sustains vast numbers of animals ; 

 the great diversity of conditions causes wide sj)ecific variation ; while 

 TOL. xvnr. 50 



